1. Field of the Invention
The present invention relates to a film useful as various optical films including optical compensation film used for display devices and protective film for polarizing film, a polarizing plate and a display device using the film, and a method of manufacturing the film.
2. Description of the Related Art
In continuous manufacturing of a long film, molecules composing the film generally align in the machine direction (MD) of the film, so that the slow axis in plane generally appears in the direction of molecular alignment or in the direction orthogonal thereto. While the molecules may otherwise be aligned in the direction (TD) orthogonal to the machine direction by stretching, or by appropriately selecting the material, it is general that also the slow axis in plane of the thus-manufactured film appears in the direction of molecular alignment or in the direction orthogonal thereto. On the other hand, there is also a strong need for films having the slow axis in plane in a direction which is neither parallel nor orthogonal to the longitudinal direction. For example, one known example of circular polarizing plate is a stack of a phase difference film having a retardation in plane of λ/4 and a polarizing film, wherein the slow axis in plane of the phase difference film and the absorption axis of the polarizing film cross at 45°. If a long film having the slow axis in plane in direction which is 45° away from the longitudinal direction is successfully provided, the film may be stacked with a long polarizing film having the absorption axis in a direction which is parallel to the longitudinal direction in a roll-to-roll manner, which largely contributes to improve the productivity.
For the purpose of manufacturing a film having the slow axis in a direction which is neither the MD nor TD, a method having been widely adopted is such as obliquely stretching the film so as to align the major constituent polymer in the direction of stretching, to thereby induce the slow axis in plane in the direction parallel to or orthogonal to the direction of alignment. For example, the above-described phase difference film typically having the slow axis in plane laid 45° away from the longitudinal direction may be manufactured by stretching in a 45° direction (see Japanese Laid-Open Patent Publication No. 2002-22944, for example). The film uniaxially stretched in the MD or TD so as to induce the slow axis in plane inparallel or orthogonal to the direction of alignment of the polymer molecule, however, tends to produce wrinkle when it is pasted to other component such as polarizing film, and may degrade the productivity.